An investigation of the fertilizing characteristics of pyriform-shaped bovine spermatozoa

Two breeding trials were done to determine the effect of pyriform-shaped bovine spermatozoa on fertility. In breeding trial (1), heifers were superstimulated by injecting follicle-stimulating hormone (FSH) and randomly allotted to an affected Bull A (n=21) or a Control Bull, C (n=18). Semen from Bull A contained 85% pyriform spermatozoa while that from the Control Bull C contained 91% normal sperm and 2% pyriform spermatozoa. Fertilization rate was lower (P=0.01) for Bull A (total of 63 ova/embryos; 68.5%) than for the Control Bull, C (total of 81 ova/embryos; 84.4%). In breeding trial (2), 37 oestrus-synchronized heifers were randomly allotted to Bull A (n=19) or Bull C (n=18). Pregnancy rates at Day 60 (37% and 61% for Bulls A and C, respectively; P=0.22) and rate of embryo/fetal loss between Days 22 and 60 of pregnancy (23% and 8% for Bulls A and C, respectively; P=0.55) were not different. In vitro experiments involving the same Bull A and another Control Bull, B, were designed to explain the mechanism of infertility caused by pyriform sperm. The mean (±SEM) number of sperm bound to the zona pellucida was lower (P<0.05) for Bull A (24.6±1.2) than for the Control Bull, B (46.6±1.9) and the percentage of zonae penetrated by sperm from Bull A (56.0%) and Bull B (82.8%) was also different (P<0.05). The percentage of pyriform sperm from Bull A bound to (53%), and penetrating (49%) the zona pellucida was lower (P<0.05) than that in the inseminate (85%). Although fertilization rates (64.1% and 72.8%) were not different (P=0.5), cleavage rates (48.2% vs. 74.1%) and morula production rates (24.8% vs. 37.7%) were less (P<0.05) for Bull A than for Bull B, respectively. In summary, pyriform sperm had reduced capability to bind to and penetrate the zona pellucida, and zygotes (resulting from the fertilization of oocytes by pyriform sperm) appeared to have a reduced ability to initiate cleavage.